Mytotonic dystrophy 1 (DM1) is a prevalent neuromuscular disorder that is caused by a poly-CTG expansion in the 3'-untranslated region of the myotonic protein kinase gene (DMPK). This disease cannot be cured, and there are no treatment options that delay disease progression. There is now strong evidence supporting a toxic gain-of-function role for the poly(CUG)RNA sequences (that result upon transcription of the poly-CTG expansion) as the molecular basis for DM1. The poly(CUG)RNA binds to and sequesters important proteins, inhibiting their normal function. Chief among these proteins is muscleblind-like protein 1 (MBNL1), which acts to mediate proper pre-mRNA splicing of several important client pre-mRNAs. As a result of the sequestration of MBNL1 by poly(CUG)RNA, fetal splice variants of at least six key proteins are produced in the adult, leading directly to the disease symptoms. There is considerable genetic evidence that disruption of the MBNL1-poly(CUG)RNA interaction will reverse DM1. Thus, the overall goal of this proposal is to identify and develop small molecules capable of disrupting poly(CUG)RNA-MBNL1 interactions and reversing the DM1 disease phenotype. The key to this approach is that the small molecules must target the RNA, rather than MBNL1, because MBNL1 must be able to act normally in the cell. The poly(CUG)RNA is a good target for the development of small molecule binders because poly(CUG) forms a structured, stable hairpin and CUG repeat sequences are not part of the normal functioning of the cell. Thus, specific targeting of this RNA sequence should disrupt interactions with MBNL1 without affecting other essential cellular processes. The specific aims of the proposal are: 1) to probe the binding affinity and specificity of MBNL proteins for poly(CUG)RNA, 2) the identification of small-molecule binders of poly(CUG)RNA, and 3) the assessment of compounds that disrupt the poly(CUG)RNA-MBNL1 interaction in vitro and in cell culture models of myotonic dystrophy. By pursuing these aims, our goal is to rapidly validate poly(CUG)RNA as a target for the treatment of DM1, and to identify compounds that will be appropriate for medicinal chemistry optimization and evaluation in pre-clinical models of DM1.